1. Field of the Invention
This invention relates to a method for the preparation of sorbents for treatment of industrial effluent gases, and, more specifically, this invention relates to a method for producing sorbents to aid in the removal of carbon dioxide (CO2) from the atmosphere or from other sources such as power plants.
2. Background of the Invention
Fossil fuels supply more than 98 percent of the world's energy requirements. The combustion of fossil fuels, however, is one of the major sources of the greenhouse gas, CO2. The ability to efficiently and safely absorb CO2 is important in the development and application of cost-effective technologies for CO2 removal from gas streams.
Separation and capture processes of CO2, as those processes relate to ocean and/or geologic sequestration, have been identified as a high-priority topic in the government.
The costs of separation and capture, including compression to the required CO2 pressure for the sequestration step, are generally estimated to comprise about three-fourths of the total cost of ocean or geologic sequestration. An improvement of the separation and capture of CO2 will reduce the total cost required for sequestration.
CO2 absorption processes using aqueous amine solutions facilitate the removal of CO2 from, gas streams in some industries. These processes often are referred to as wet chemical stripping.
Wet chemical stripping of CO2 involves one or more reversible chemical reactions between CO2 and amine substances to produce a liquid species, such as a carbamate. Upon heating, the carbamate breaks down to free. CO2 with the original amine regenerated to subsequently react with additional CO2.
Typically, the amines, monoethanolamine (MEA) and diethanolamine (DEA), are used as 25 to 30 wt. % amine in aqueous solution. The amine solution enters the top of an absorption tower while the carbon dioxide containing gaseous stream is introduced at the bottom. During contact with the CO2-containing gaseous stream, the amine solution chemically absorbs the CO2 from the gaseous stream to create a carbamate. Conversion of carbamate ion back to CO2 proceeds through a thermal regeneration process, typically at a temperature of about 120° C. Carbon dioxide and water emerge from the amine solution and the water is separated via condensation using a heat exchanger. After regeneration, the amine solution is recycled back to the absorption tower for additional CO2 absorption.
Carbon dioxide capture in the above-described manner is energy intensive. In addition, the amine process is restricted to CO2 capture at ambient temperatures. Further, the amine solution has a limited lifetime due to degradation through oxidation of the amine. In addition, high amine concentrations and high CO2 loadings exacerbate corrosion problems of process equipment. The CO2 capture capacity of commercial amine processes is about 0.68 mole/kg. Since the capacity is low, either large reactors or frequent regeneration is necessary.
CO2 capture capacity via the Selexol process is about 0.16 mole/kg. The Selexol process utilizes dimethylether of polyethylene glycol as a solvent for the physical absorption of carbon dioxide at high carbon dioxide pressures. The solvent is regenerated using pressure reduction. Current commercial processes for CO2 removal, such as the Selexol process, require cooling of the gases to ambient temperature before utilization of said processes.
Solid sorbents serve as alternatives to CO2 capture utilizing liquids. For example, pressure swing adsorption-absorption/temperature swing adsorption-absorption (PSA/PTA) processes are more suitable for CO2 capture from coal gasification.
PSA/PTA systems can be even more energy efficient for CO2 capture in integrated gasification combined cycle (IGCC) processes, if only the sorbents were operational at moderate or high temperatures. Currently, water-gas shift reactors in IGCC processes elevate gas streams to 300° C., so as to convert IGCC gas to CO2, H2, and steam. These gases must be cooled before currently available CO2 removal technology can be used.
Thus, there are considerable advantages of developing sorbents for CO2 capture at moderate to hot gas temperatures inasmuch as the hot gas remaining after CO2 removal can be directly introduced to the turbine systems. If CO2 can be removed from the gas stream directly after the water-gas shift reactor, a pure H2 stream can be obtained at high temperatures for various applications. Aside from use in IGCC applications, sorbents for moderate to hot gas temperatures also can be useful for chemical and metallurgical applications. Yet, there are no known regenerable sorbents which can be used easily and effectively for CO2 capture at the temperature range of 100° C. to 500° C.
U.S. Pat. No. 6,322,612 awarded to Sircar, et al. on Nov. 27, 2001 discloses a process for the removal of bulk carbon dioxide from wet, high-temperature gases.
U.S. Pat. No. 5,917,136 awarded to Gaffney, et al. on Jun. 29, 1999 discloses a carbon dioxide pressure swing adsorption process using modified alumina adsor-bents. CO2 sorption capacities of 0.11-0.29 mole/kg have been obtained with these materials.
U.S. Pat. No. 5,214,019 awarded to Nalette, et al. on May 25, 1993 discloses a process to enhance carbon dioxide sorption rates by the use of hygroscopic additives.
The aforementioned patents report processes using modified alumina, hydrotalcite, and double salts to remove CO2 in the temperature range of 100° C. to 500° C. However, the CO2 sorption capacities of these processes, as determined by thermogravimetric analyses (TGA) utilizing milligram-level quantities, are very low. The results of flow reactor studies have not been reported.
None of these patents disclose an effective and regenerable sorbent for CO2 capture in the temperature range of 100° C. to 500° C., nor any method or process for fabricating such a sorbent. In addition, these patents do not disclose the use of a sorbent containing a combination of alkali compounds with alkaline-earth metal oxides and/or alkaline-earth metal hydroxides.
A need exists in the art for a process to produce sorbents with wide capabilities in warm- and hot-gas cleanup. In addition, the sorbent should be easily regenerated at higher temperatures for use in additional sorption/desorption cycles. Finally, the materials used in sorbent preparation should be inexpensive, and should have very high CO2 sorption capacities over the prior art.